The wire electric spark cutting method is indispensable in the manufacture precision dies. In this method, a wire pulled with a prescribed force is moved in a direction intersecting a workpiece while spark discharging is applied to the workpiece. At the same time, the workpiece is moved in accordance with the desired shape to be processed.
Generally, a brass wire has been used as the electrode wire for electric spark cutting, (hereinafter referred to as cutting wire.
The cutting wire should have the following characteristics. Namely,
(1) it must be capable of improving the processing speed;
(2) sticking material is not generated;
(3) it has a high accuracy of processing;
(4) it has a superior straightness; if the wire has superior straightness, the cutting wire will not be twisted or curled, so that the accuracy in processing can be improved and the number of failures due to breaking of the cutting wire can be reduced;
(5) the cross sectional shape of the cutting wire is close to a true circle; this improves the processing accuracy processing;
(6) it has a high tensile strength; which prevents a breaking of the cutting wire; and
(7) it is conductive.
The following two metals satisfy the above described conditions.
One is pure tungsten. The tensile strength of the pure tungsten is approximately triple that of brass. Therefore, when the pure tungsten is used as a material for making the cutting wire, the diameter of the cutting wire can be reduced. Therefore, when a pure tungsten cut wire is and a more precise processing becomes possible.
Another metal for these purpose is pure molybdenum. The tensile strength of the pure molybdenum is approximately 1.6 times that of brass. Therefore, when the pure molybdenum is used as the material for the cutting wire, the diameter of the cutting wire can be reduced and a more precise processing becomes possible.
However, a cutting wire made of pure tungsten or of pure molybdenum has the following two drawbacks. First, the pure tungsten cutting wire and the pure molybdenum cutting wire have an inferior discharging capability, so that sparks are not very satisfactorily generated. Therefore, the processing speed becomes slow and the accuracy of the cut surface of the workpiece becomes inferior. The number of failures due to breaking of the cutting wire is increased.
The second drawback is as follows. The pure tungsten cutting wire and the pure molybdenum cutting wire a lower tensile strength at high temperature, so that when strains are generated in the wires at high temperature. The accuracy of the cut surface of the workpiece is reduced.